Applicants' invention relates generally to devices for protecting electrical equipment and to methods of making such devices, which devices are commonly referred to as "surge protection" or "transient voltage suppression" devices. Transient voltage protection devices were developed in response to the need to protect the ever-expanding number of electronic devices upon which today's technological society depends from high voltages. Electrical transient voltages can be created by, for example, electrostatic discharge or transients propagated by human contact. Examples of electrical equipment which typically employ transient voltage protection equipment include, telecommunications systems, computer systems and control systems.
Recent developments in transient voltage protection technology have centered around usage of a material having a variable impedance which interconnects, for example, a signal conductor with a ground conductor. The variable impedance material exhibits a relatively high resistance (referred to herein as the "off-state") when the voltage and/or current passing through the signal conductor is within a specified range, during which time the signal conductor is ungrounded.
If, however, the signal conductor experiences a voltage which exceeds the threshold for which the variable impedance material (and the transient voltage protection device generally) has been designed, then the electrical characteristics of the variable impedance material will change such that the material exhibits a relatively low impedance (referred to herein as the "on-state"). At this time, the pulse or transient voltage experienced by the signal conductor will be shunted to the ground conductor, and the voltage associated with the pulse will be clamped at a relatively low value for the duration of the pulse. In this way, the circuitry associated with the signal conductor is protected.
The variable impedance material will recover after the voltage or current pulse has passed and return to its high impedance state. Thus, the signal conductor and associated circuitry can continue normal operation shortly after the pulse has ended.
Different types of variable impedance materials, also sometimes referred to as "overstress responsive compositions", are known in the art. These materials can, for example, be fabricated as a mixture of conductive and/or semiconductive particles suspended as a matrix within a binding material, which can, for example, be an insulative resin. Numerous examples of these types of materials can be found in the patent literature including U.S. Pat. Nos. 5,393,596 and 5,260,848 to Childers, U.S. Pat. Nos. 4,977,357 and 5,068,634 to Shrier and U.S. Pat. No. 5,294,374 to Martinez, the disclosures of which are incorporated here by reference. U.S. Pat. Nos. 3,685,026 and 3,685,028 also disclose compositions including conductive particles dispersed in a resin.
U.S. Pat. No. 5,278,535 to Xu et al. describes an electrical overstress pulse protection device which employs a variable impedance material. Specifically, Xu et al. provide a thin flexible laminate for overlay application on the pins of a connector. The laminate includes an electrically insulating substrate, a conductive lamina of apertured pin receiving pads, a separate ground strip adjacent the pads, and an electrically insulating cover. An electrical overstress pulse responsive composite material is positioned such that it bridges the pads and the ground strip.
This patent to Xu et al., however, uses conventional semiconductor fabrication techniques to create the pulse protection device including forming the substrate from a conventional resin material, e.g., of the type typically used for substrates of printed circuit boards. Similarly, Xu et al. describe forming the conductive elements using etching techniques, which are also well known in the semiconductor fabrication. While these techniques may be appropriate when working with thin film metal conductors, Applicants have determined that other techniques and materials are more desirable when manufacturing signal and ground conductive elements having a greater thickness, e.g., on the order of 0.5-1.0 mils, or more.